This invention relates to air conditioning systems employing mechanical refrigeration units, and in particular to such a system having improved dehumidification capabilities.
It is well recognized that in many geographical areas, a proper air conditioning system should not only lower the temperature of the space being served when the temperature therein has exceeded a predetermined level, but should also decrease the relative humidity of the space even though the temperature therein may not have exceeded the predetermined level. Very often, the typical air conditioning system relies solely on temperature reduction whereby the conditioned air temperature is lowered beneath its dew point to achieve any absolute humidity reduction. If the dew point is not reached, the mere lowering of the temperature of the conditioned air will actually result in an increase in the relative humidity. High relative humidity creates not only human discomfort, but in addition permits mildew and other types of fungi to form on furniture, clothing, etc.
It has been recognized that a lowering of the cooling or evaporator coil temperature will not only result in an increased sensible heat reduction, i.e. a reduction of the conditioned air temperature, but will also result in an increase in the latent heat reducing capabilities of the system, i.e. removal of water vapor from the air. The increase in the latent heat reducing capabilities in proportion to the accompanying increase in sensible heat cooling will be even greater.
A variation in the velocity of air across the cooling coil or evaporator will also vary the proportion of sensible heat reduction to latent heat reduction. In effect, by lowering the flow rate of the air across the coil, a reduction will occur in the sensible heat load imposed on the system; that is, less heat will be transferred to the refrigerant of the system thereby lowering its pressure and temperature. As noted before, lowered refrigerant temperature results in increased dehumidification capabilities for the system. The reduced air flow rate results in the air temperature decreasing to a lower level. The lower temperature level approaches the temperature of the refrigerant flowing through the evaporator coil whereby the moisture content of the air is further reduced due to the lower dew point. Hence, various arrangements have been proposed for lowering the flow rate of the air across the cooling coil when dehumidification is required in the absence of a need for sensible heat reduction. The reduced air flow results in a closer approach of the dew point temperature of the effluent air to the refrigerant temperature within the coil thus further reducing power requirements for dehumidifying.
Additionally, by reducing the speed of the fan provided to route air in heat transfer relation with the evaporator and thus lowering the temperature of the refrigerant flowing therethrough, the power consumed will be substantially reduced in removing the same amount of moisture from the air. It has been mathematically determined that a reduction in refrigerant temperature from 44.degree. F to 30.degree. F will result in almost a 300% increase in the amount of moisture that may be removed for the same amount of sensible heat cooling. Thus, to remove the same amount of moisture, an air conditioning system operating at the lower temperature need operate for only one third the period of time when compared to a system operating at the higher refrigerant temperature. When operating in a dehumidifying mode with the air conditioning system operating for example only one third the time, the dry bulb temperature may rise. A rise in dry bulb temperature will actually result in a decrease in the relative humidity, thereby further decreasing the need to operate the air conditioning system to maintain a desired relative humidity. Since, it is contemplated that the dehumidifying mode of operation will be typically employed when the air conditioned space is unoccupied, the rise in dry bulk temperature will not cause human discomfort. If the dry bulb temperature is allowed to rise when on the dehumidifying mode, such rise in dry bulb temperature further reduces the relative humidity resulting in much less power consumption as compared to contemporary practice, in order to maintain the same relative humidity.
Examples of the prior art are illustrated in U.S. Pat. Nos. 2,236,058; 2,296,530; 2,685,433; and 3,251,196. While each of the prior art systems reduce the flow rate of the air when relative humidity reduction is desired, each of the systems illustrate complex arrangements for achieving the foregoing. In addition, none of the systems directly or indirectly monitor the temperature of refrigerant in the evaporator, and if the temperature of the refrigerant is reduced below a predetermined level, moisture or condensate formed thereon will freeze. The formation of large quantities of frost on the surface of the evaporator coil reduces the efficiency of operation of the air conditioning system or renders it inoperable and is therefore undesirable.